One way scientists can work out how threatened a species is, is to measure the birth and death rates. If a species becomes small in numbers, it can get to a critical point. This happens when there are not enough individuals left alive to continue the species (critical depensation).

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Under the Endangered Species Act in the United States, "threatened" means any species which is likely to become endangered in the near future, in all, or a large part of its range.[1] The Bay checkerspot butterfly (Euphydryas editha bayensis) is an example of a threatened species protected by the Endangered Species Act.